Generally, a separation membrane used for the preparation of ultra highly pure hydrogen has low permeability. Hence, in order to overcome such a problem, intensive and extensive research on improvement of the selective permeability of the membrane by applying a non-porous palladium membrane on a porous support is presently being studied. The non-porous palladium membrane has high hydrogen selectivity but has low permeability. Therefore, although the selective hydrogen permeability of the separation membrane is intended to increase by coating the surface of the porous support with a thin palladium membrane, the separation membrane coated with only palladium suffers because it may be deformed due to phase change of the lattice structure while hydrogen gas is absorbed. With the goal of preventing such deformation, a palladium alloy separation membrane is mainly used at present.
A metal, which is alloyed with palladium, includes, for example, silver, nickel, copper, ruthenium, molybdenum, etc. In particular, a palladium-copper alloy membrane, which is prepared using inexpensive copper, has resistance to hydrogen sulfide and sulfur compound poisoning superior to other palladium alloy membranes, and thus has been thoroughly studied in recent years. In such cases, the alloy membrane is typically prepared by alloying a copper plating layer and a plated palladium layer (or a sputtered palladium layer) sequentially coated on a porous ceramic support or a porous metal support. However, the palladium-copper alloy membrane prepared using such a conventional method is disadvantageous because it is not dense and has fine pores or defects therein, thus having low hydrogen selectivity (FIG. 1). Further, when the copper layer, serving as an alloy source, is present as an intermediate layer between the support and the palladium layer, it may be separated due to the thermal diffusion and fluid reflow properties at a usage temperature of 500° C., therefore negatively affecting the adhesion. Consequently, the palladium-copper alloy separation membrane breaks.
Turning now to FIG. 2, there is illustrated a palladium-copper alloy membrane, which comprises a palladium-copper alloy coating layer provided on a porous metal support by sequentially forming a nickel plating layer as an underlayer of a copper plating layer, a copper plating layer and a palladium plating layer on the porous metal support and then heat treating them. In addition, this drawing shows the result of heat treatment for the alloy membrane at a usage temperature of 500° C. for 100 hr.
From the surface microstructure of the separated upper portion of the alloy membrane and the EDS result shown in FIG. 2, it can be seen that the microstructure of membrane is not dense and copper and palladium are present in this portion. FIG. 3 illustrates the surface microstructure of the separated lower portion of the alloy membrane and the EDS result, in which the microstructure of membrane is not dense and copper and nickel are present in this portion. Thereby, it appears that the copper plating layer is separated through the thermal diffusion of copper atoms and moved to the upper layer (palladium coating layer) and the lower layer (support) of the copper plating layer.
Recently, a palladium alloy composite separation membrane has been developed using a porous metal support made of stainless steel through an electroplating process. However, since the pore size of the porous stainless steel support used is large and the surface thereof is rough, a complicated pretreatment procedure is required to apply the palladium alloy separation membrane. In the case where the electroplating process is conducted on the porous stainless steel support to form a palladium alloy coating layer, the support may be corroded by hydrochloric acid acting as a main component for activation of a plating process, and hydrogen separation properties may decrease due to additive impurities in a plating solution. In addition, the palladium metal is diffused into the support at a usage temperature of 500° C., thus decreasing durability. As well, upon reforming of hydrogen gas, hydrogen brittleness of a stainless steel substrate is caused by the hydrogen absorption, and thus the substrate may break.